Heart failure is a major disease with five million patients in the United States alone and tens of millions worldwide. The individuals at risk of heart failure are estimated at 60 million in the United States only; one million are hospitalized, the rest being in the care of heart failure clinics. Basic information about the heart is needed in the heart failure clinics or general practitioners' offices for patient management. This information includes images as well as quantification data, such as ejection fraction, computed from the image once the image is acquired. Ultrasound is a reliable and cost-effective imaging modality for soft tissue such as the heart.
Acquisition of an ultrasound image requires a skilled sonographer. One parameter the sonographer, or other clinician trained in sonography, optimizes is the field of view. The apical four chamber view is a standard one for routine cardiac checkups. The clinician places the head of the ultrasound probe, or “transducer probe”, on the patient. An effective site on the patient's skin for placement of the probe for various views is part of the clinician's training, and the site can vary from patient to patient. For the apical four chamber view the probe is placed over the apex of the heart. The probe also needs to be manually tilted, typically in different directions until the organ is captured for imaging. This is all done interactively, with the clinician viewing the image, which is usually a sonogram, on-screen. Interpreting a sonogram is a skill that must be developed, e.g., through training and practice. The clinician's experience tells him or her, in an ongoing iterative process, how to shift and tilt the probe to achieve an effective acoustic window.
Echocardiography is challenging as the heart is surrounded by ribs and lung tissue. Ultrasound can hardly penetrate calcified ribs (typically encountered in the apical view) and lung tissue because of severe acoustic impedance mismatch between them and other soft tissues. In addition, ultrasound absorption in ribs is quite high compared to tissue. Conventionally, optimization of ultrasound image quality is done solely by the user based on real-time-displayed grayscale ultrasound images on the screen. Though experienced users are usually capable of recognizing image degradation and improving image quality accordingly by moving the probe to a better position, less experienced users might acquire compromised images because of inferior hand-eye coordination and less awareness of artifacts. Successful ultrasound scanning strongly relies on training and experience of the user.
When imaging from the apical view of the heart, a standard view, the ultrasound probe has to be placed in the right intercostal space based on the user's expertise to avoid blockage due to calcified ribs. An experienced user also makes sure lung tissue does not get into the way.